1. Field of the Invention
The invention relates to an arrangement for adjusting the width of a fibre web when water is removed from pulp suspension in an apparatus comprising two filter wires permeable to water, the filter wires having the same direction of travel and both wires being arranged to be supported by surfaces equipped with water openings and to form therebetween a gap narrowing in the direction of travel, and side parts located on both edges of the gap to prevent the pulp suspension from being extruded out of the gap in the crosswise direction thereof, the openings in the upper and lower surfaces of the gap being formed such that the both surfaces comprise a solid edge part next to the side parts such that the edges of the wires extend over said solid edge parts, and means for feeding the pulp suspension into the gap between the wires in the inlet direction thereof.
2. Description of Related Art
When water is removed from pulp suspension when, for example, ash or other impurities are washed off it, the pulp is usually fed to a gap-like space confined by wires. Behind the wires, there are supporting surfaces equipped with holes or other openings, whereby the water can be removed via the wires and further via the openings thereof. It is also known that openings cannot be used in the supporting structure at the edges of the wires running in the gap but the supporting structure must have a solid section at the edge part of the gap in order to prevent the pulp from penetrating under the edges of the wire. On the sides of such a gap there are also provided side plates to close the gap such that the pulp suspension is not extruded out of the gap to the sides thereof but proceeds towards the outlet opening of the gap in the direction of travel of the wire. The gap is shaped so as to narrow in the direction of flow of the pulp suspension such that the pulp suspension is pressed against the wires; consequently, the water flows through the wire. The problem with such an apparatus is that the pulp suspension tends to become pressed to the edges of the wire and further under the edges thereof, thereby dirtying the edges of the wires and, furthermore, the pulp suspension splatters all over at the outlet opening of the gap, dirtying the apparatus. In order to prevent the above, it is known to use water jets to wash the pulp off the edge of the wire and thus to prevent the edge of the wire from becoming dirty and the pulp from being splattered all over. This solution is, however, rather difficult since using water jets necessarily causes splashes and it is difficult to collect the water and the fibre material removed by the water jets.
Attempts have also been made to utilise various sealing elements to block the gap between the wires so that the pulp suspension cannot be extruded from the edge. The problem with this solution is that the wires equipped with seals are extremely expensive and, furthermore, their service life is quite short. The use of separate seals, in turn, has proved to be difficult in practice since the pulp suspension penetrates extremely easily between the wires and the seals before the actual sealing is performed. Controlling the separate seals has also proved to be difficult, and the seals tend to wear quickly.
An object of the invention is to provide a solution by which the disadvantages of the known solutions are avoided and which is easy to implement. An arrangement of the invention is characterized in that the arrangement comprises means for feeding water to the sides of the gap such that while flowing together with the pulp suspension towards the outlet opening of the gap, the supplied water pushes the pulp suspension towards the centre of the gap at least substantially as far as to the openings in the upper and lower surfaces.
The basic idea of the invention is that such an amount of water is fed into the gap to the sides thereof that the water is pressed towards the centre of the gap as the height of the gap decreases, the water simultaneously pushing the pulp suspension away from the edge of the gap, whereby the web that exits from the chamber between the two wires is narrower than the wires, the edge of the wire becoming substantially purified of pulp. The advantage of the invention is that an extremely small amount of water can push the pulp from the edge of the gap towards the centre thereof. The supplied water, which retains its volume, penetrates towards the centre of the gap as the height of the gap decreases, simultaneously narrowing the web until the openings in the supporting structure on the other side of the wires allow excess water to be removed from the gap; consequently, the pulp remains on the wire substantially along the entire width between the extreme openings. The water not only pushes the pulp from the edge of the gap towards the centre thereof but also washes the edges of the wire substantially clean, thereby substantially preventing splashing at the outlet end of the gap or the apparatus from becoming dirty. The feeding of water in accordance with the invention is also extremely easy to implement by placing only one or more water feeding hoses or channels in the side plates of the gap, for example; no other separate equipment is necessary.